A standard self-closing valve such as used in a toilet flusher or in a faucet normally has a main valve having a seat between a high-pressure input conduit and a low-pressure output conduit, a main valve element movable into and out of engagement with the seat to block and permit flow from the input conduit to the output conduit, and a release chamber behind the main valve element pressurizable to press the main valve element against the seat. A small-diameter passage extends from the input conduit to the chamber so that the chamber can be pressurized via the passage from the input conduit to hold the valve element against the seat. A pilot or control valve connected between the chamber and the output conduit can be opened to depressurize the chamber and thereby lift the valve element off the seat.
This simple system therefore allows the valve to be opened suddenly to allow a rush of water past the seat. As the chamber repressurizes via the throttle passage, the main valve body moves back to the closed position. The main problem with this system is that, if the throttle passage has too small a flow cross section, the valve stays open much too long while if it is too large the valve closes brusquely, creating noise and a pressure shock in the line.
Accordingly it has been suggested in German patent 2,603,731 of Blanke provides at the mouth of the throttle passage a special throttle valve having a throttle piston having an outer face exposed to pressurized liquid in the input conduit and an inner face turned toward a mouth of the passage and provided with a throttle rod with a tapered tip engageable in the passage mouth. A spring urges the piston outward, away from the passage, and orifices in or around the piston allow liquid to pass through it. Thus the tapered end will initially be engaged deeply in the passage mouth, wholly or substantially blocking it, until the pressure differential across the piston lessens, whereupon it will pull back and allow increased flow through the passage to the release chamber.
Such an arrangement does indeed provide a somewhat smoother closing of the valve, but has several disadvantages. First of all its structure is fairly bulky, making it unsuitable for installation right in an automatic-shutoff faucet of the type used in commercial establishments. It also is prone to jamming, as the small gap between the rod tip and the mouth can catch foreign bodies, blocking the piston's movement and rendering the throttle valve ineffective.